A few days after the San Bernardino shootings I attended a Council of Foreign Relations luncheon headlined by William Perry, the former defense secretary under Bill Clinton. In the wake of the Paris attacks, he had a less than cheery assessment of the world's nuclear dangers that lost the appetites of most of us in attendance. In particular, for anyone living in New York or Washington DC he had a special warning, there is a high likelihood that a terrorist will set off a dirty bomb in either of these cities within the next year. Such a bomb could make uninhabitable for decades a one square mile section of the city and the knowhow to set off such a device is well within the capacity of various terror groups, he said. With that, just about everyone took a pass on the chocolate mousse.

That night I went to the movies near Lincoln Center and walked back to the East Side surveying the endless number of crowded "soft targets" offered along NY streetscapes to any diabolical person with a readily available assault weapon at his or her disposal. Forget about the obvious places like Times Square or the Rockefeller Center Christmas tree in midtown (which now happen to be guarded by seeming brigades of machine gun toting police), these Upper West streets were full of easy marks--people headed to the concert halls and diners seated in rows of restaurants and bars along Columbus and Amsterdam Avenues. Then what about Greenwich Village or Williamsburg in Brooklyn or countless other gathering spot neighborhoods throughout the city--the crowds they attract are also vulnerable to chaos inducing attacks.

Back in the post-white flight era of 1970s and 1980s, the talk was America's major cities were doomed by ever lurking street violence and crime waves. Of course, the stanching of murder rates and improved policing has been essential to spurring the country's more recent urban renaissance. Safe streets and exuberant throngs have buried most concerns since—tourists continue to flow into New York, DC, and San Francisco in record numbers and the millennial/baby boomer move-back in trend maintains its pace.